<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        * {
            margin: 0;
            padding: 0;
        }
        div {
            width: 600px;
            margin: 100px auto;
        }

        .message {
            display: inline-block;
            font-size: 12px;
            color: black;
            background: url(../images/open.png) no-repeat;
            padding-left: 20px;
        }

        .wrong {
            color: red;
            background-image: url(../images/close.png);
        }

        .right {
            color: chartreuse;
            background-image: url(../images/open.png);
        }
    </style>
</head>
<body>
    <div class="register">
        <input type="password" class="ipt" name="" id="">
        <p class="message">请输入6~16位密码</p>
    </div>
    <script>
        var ipt = document.querySelector('.ipt');
        var message = document.querySelector('.message');

        ipt.onblur = function() {
            if(this.value.length <6 || this.value.length>16) {
                message.className ='message wrong'
                message.innerHTML = '您输入的位数不对'
            }else {
                message.className = 'message right'
                message.innerHTML = '输入正确'
            }
        }
    </script>
</body>
</html>